1. Field of the Disclosure
Generally, the present disclosure relates to tongs for making up and breaking out threaded joints. More specifically, the present disclosure relates to a method and apparatus for calibrating tong torque.
2. Description of the Related Art
Making up (tightening) and breaking out (loosening) threaded joints between tubular products (tubulars) are important operations in the various industrial applications involving the transfer of fluids. In oilfield applications, for example, it is quite common to make up (tighten) and/or break out (loosen) the threaded joints between various types of tubular products, such as tubing, casing, drill pipe, and the like. In some applications, threaded joints between tubulars may be made up to form lengths of tubular products (tubular strings) that are sufficient to perform operations such as the drilling of a borehole or the production of fluid from a borehole, and the like. In order to make up or break out a threaded joint between two tubulars, a “backup” tong engages a first of two tubulars, and a “power” tong engages a second of the two tubulars. The backup tong is adapted to hold the first tubular in relatively firm manner, while the power tong is adapted to apply torque to the second tubular so as to rotate it relative to the first tubular held by the backup tong. The direction of the torque applied to the second tubular by the power tong indicates whether a threaded joint is being made up (tightened) or broken out (loosened).
Power tongs used to apply torque to tubular products during make up and/or break out are typically classified as “open-head” or “closed-head.” An “open-head” power tong has a central opening and a side opening providing a passage to the central opening. The side opening of an open-head power tong is sometimes referred to as the “throat.” Both the central opening and the throat are large enough to receive a tubular, and the throat permits the open-head power tong to engage tubular by allowing the tubular to pass sideways through the throat and into or out of the central opening—i.e., horizontally. On the other hand, a “close-head” power tong only has a central opening, and does not have a throat to permit the sideways movement of the tubular into the central opening. Therefore, a closed-head power tong can only engage a tubular by allowing the tubular to pass into and out of the central opening through the top or the bottom of the closed-head power tong—i.e., vertically. Some aspects of various prior art power tongs are illustrated in FIGS. 1 and 2, which will now be discussed.
FIG. 1 shows an illustrative embodiment of a prior art open-head power tong from U.S. Pat. No. 4,170,907 (issued to Cathcart). It should be noted that the reference numbers used in Cathcart have not been preserved in FIG. 1, so as to avoid any duplication in numbering with the embodiments illustrated in the present disclosure. One embodiment of an open-head power tong 1 disclosed in Cathcart includes a bifurcated frame 2 defining a central opening 3 and a side opening 4. As shown in FIG. 1, jaws 5 are disposed within the central opening 3 for engaging a drill pipe, that is, when the drill pipe is positioned within the central opening 3. The jaws 5 are driven through a drive train (not shown) by a hydraulic motor 6, which receives fluid pressure from a hydraulic pump (not shown) through a hydraulic control valve 7. The hydraulic control valve 7 is movable between three spool positions: 1) a first position that drives the motor 6 in a clockwise direction; 2) a second position that drives the motor 6 in a counterclockwise direction; and 3) a third position that places the motor 6 in neutral. A door 8 is provided at the side opening 4 to control access to the central opening 3. Typically, the door 8 is closed while operating the power tong so as to protect the operator of the power tong from the moving jaws 5.
FIG. 2 shows an illustrative embodiment of a prior art open-head power tong from U.S. Pat. No. 4,445,403 (issued to Janzen et al.) As shown in FIG. 2, the power tong has a frame 10 defining a throat 20 for receiving a pipe (not shown). A circular opening 21 is provided in the center of the frame 10, and a pipe can pass through the throat 20 into the circular opening 21. A pair of arcuate bearing and guide segments 22, 24 is mounted on opposite sides of the throat 20 and a drive ring 38 is mounted for rotation relative to the frame 10. The drive ring 38 has an opening 40 that is of substantially the same size as the throat 20 and that is aligned with the throat 20. The drive ring 38 is guided along its outer periphery and retained within the frame by the bearing and guide segments 22, 24. Gear teeth (not shown) are secured to a projection that extends radially from the outer circumference of the drive ring 38. The gear teeth mesh with rotary idler gears 88 and 90 of a drive train 52 that is powered by a motor (not shown). Although not shown in FIG. 2, the power tong also includes a die carrier with means for gripping a pipe placed in the circular opening 21. When the drive ring 38 rotates, the die carrier also rotates, and cam action between the drive ring 38 and the die carrier rotates the means for gripping a pipe in contact with a pipe received in the circular opening 21.
In general, when making up a threaded joint, the torque applied to the tubular by the power tong should not be too high, as the threads may become overstressed and possibly even damaged. On the other hand, the applied torque should not be too low, as the threaded joint may leak and/or become loose during operation. Additionally, excessive torque that may be applied to tubular products, either when making up or breaking out threaded joints, may also damage the surfaces of the tubulars. Thus, monitoring or measuring the amount of torque applied by the power tong during a make-up or break-out operation may be an important component of operating the power tong. However, the equipment that is commonly available for measuring the make-up or break-out torque of a power tong can be expensive to buy, or to rent, and cumbersome to use. For example, in some prior art systems, a hydraulic load cell is positioned in a line extending from the power tong to a fixed point. As the tubular goods are being made up (or loosened), the hydraulic load cell measures the torque being applied on the tubular goods that are currently in the tong. The measurement of the torque is read from the hydraulic load cell, and it can be read either manually or automatically. This hydraulic load cell technique is typically applied when making or breaking every connection.
Consequently, in some operations, operators may choose to forego direct measurement of tong torque, and instead rely on secondary indications of applied torque, such the pressure reading of a pressure gage used to monitor hydraulic pressure in a hydraulic line connected to the tong. In other cases, operators may not even use secondary indications of torque, such monitoring hydraulic pressure, and may instead rely on experience alone to determine whether or not tong torque is within an appropriately safe range. The latter “experience-based” approach may often lead to many, if not most, of the threaded joints being over-torqued.
Accordingly, there is a need to provide a functional, accurate, and low cost torque-calibrating device for the type of power-tong operations often performed to make up or break out threaded joints between tubular products, so as address or reduce at least some of the problems outlined above.